This invention relates to power plant operations, and, more particularly, to an approach for removing particulate matter from a flue gas stream produced in a fossil fuel power plant, especially a coal-fired power plant.
In a fossil fuel power plant, a fuel is burned in air to produce a flue gas. The flue gas heats water in a boiler to generate steam, which turns a turbine to produce power. After passing through various apparatus, the flue gas is exhausted through a stack to the atmosphere.
The flue gas of certain fossil fuels (i.e. coal) includes solid particulate matter and a variety of gaseous contaminants. The maximum permissible emission levels of the particulate matter and gaseous contaminants are set by laws and regulations. The maximum emission levels are typically far less than the amounts present in the flue gas as it is produced, and various types of gas treatment apparatus are usually provided to reduce the particulate matter and gaseous contaminants in the flue gas before it leaves the stack.
In many power plants, particulate matter in the gas stream is removed by electrostatic precipitation. An electrostatic charge is applied to the particulate matter in the flue gas, and the flue gas passes between charged electrodes. The particulate matter is deposited upon the electrode having the opposite charge to that of the particulate and is later removed.
In plants burning coal, the fuel typically contains from about 0.2 percent to about 6 percent sulfur, which at least in part oxidizes to sulfur dioxide during combustion. A small part of the sulfur dioxide further oxidizes to sulfur trioxide. Since the combustion air and the fuel also contain moisture, the flue gas contains water vapor. The sulfur trioxide and water vapor in the flue gas react to produce sulfuric acid, which deposits upon the particulate matter. The sulfuric acid deposited upon the particulate matter imparts a degree of electrical conductivity to the particulate and promotes the electrostatic precipitation process.
If the fossil fuel contains too little sulfur, so that there is a deficiency of sulfur trioxide, and thence sulfuric acid in the flue gas, the electrostatic precipitator may not function properly because of the high electrical resistivity of the particulate. It is therefore known to add sulfur trioxide from an external source to the flue gas produced frown burning low-sulfur fossil fuels. See, for example, U.S. Pat. No. 3,993,429.
In the '429 sulfur trioxide conditioning system, sulfur is burned to form sulfur dioxide, which is passed over a catalyst to achieve further oxidation to sulfur trioxide. The sulfur trioxide is injected into the flue gas flow upstream of the electrostatic precipitator. The amount of injected sulfur trioxide is controlled by varying the amount of sulfur that is burned. Other similar sulfur trioxide systems, which have been successfully used commercially, include a system which starts with a sulfur dioxide feedstock, which is vaporized and then catalytically converted to sulfur trioxide.
Sulfur trioxide injection systems, such as illustrated in the '429 patent, work well and are widely used. In some instances, however, there are drawbacks: high equipment capital costs; a constant supply of sulfur or sulfur dioxide feedstock is required, and this feedstock must be safely handled; the several components of the burning, catalyzing, and injecting system must be kept in good working order; there is a substantial power consumption associated with the process; when the plant or system goes into stand-by condition, the system, at least from the converter forward, must be purged to prevent excessive corrosion of the system and/or blockage of the probe nozzles; the injection arrangement must be operative over a range of boiler operating conditions in a manner that appropriate mixing is achieved prior to the flue gas stream entering the precipitator; because the conversion of the newly produced SO.sub.2 to SO.sub.s is not always 100% efficient, trace amounts of excess SO.sub.2 may be produced; in many instances, significant runs of hot gas insulated duct-work must be included, together with complicated and costly manifold assemblies; and the like.
U.S. Pat. No. 5,011,516 describes an alternate approach to the types of systems illustrated in the '429 patent, and teaches an arrangement wherein a slip stream of flue gas is drawn from the main flow and passed over a catalyst. A portion of the sulfur dioxide in the slip stream is oxidized to sulfur trioxide, and the slip stream is merged back into the main flue gas flow. While of interest, this approach has major drawbacks when implemented, for example: system thermal efficiency is reduced because less heat is recovered; there is typically insufficient mixing of the slip stream with the main flow at the point where they rejoin, due to an insufficient pressure differential; and the like.
Moreover, the '516 patent does not disclose any approach which permits control of the amount of sulfur trioxide produced, responsive to variations in the sulfur content of the fuel and changes in other operating parameters. A patent to a related approach, U.S. Pat. No. 3,581,463, suggests using a fan to draw a portion of the hot gas flow into the slip stream, but gives no further details as to how the amount of sulfur trioxide can be controlled. One can imagine that valving could be added to the slip stream to control its total flow, but such valves are complex, expensive, and difficult to build.
U.S. Pat. No. 5,320,052, which is assigned to the same assignee as is this invention, provides an improvement over the approaches discussed above and includes a catalytic converter support adapted to be disposed across at least a portion of the cross-section of the main duct, and a catalyst for the oxidation of sulfur dioxide to sulfur trioxide is supported by the catalyst support. This system further includes a mechanical adjustment means for selectively adjusting the amount of surface area of the catalyst which is exposed to the flow of flue gas in the main duct. While it is believed that the '052 system is an advance over the prior art discussed hereinabove, several problems and/or deficiencies may exist, for example: structural modifications to the duct, which are required in a retrofit and/or new installed FGC system of this sort, is expensive and may be difficult to achieve in many instances; mechanical complexity, with a resultant potential for breakdown; the area required of the catalyst, and the supporting structure at the face is relatively substantial and the may result in a significant back pressure being created, which in turn may result in a decrease in power plant efficiency; depending upon the various adjustments of conversion required, the catalyst will have a tendency to uneven wear; and the like.
In addition to the above approaches, a currently pending patent application, which is assigned to the same assignee as is this invention, provides for s somewhat different approach. This application includes catalyst positioned within the main flue gas duct, and incorporates heating and/or cooling means for selectively varying the surface temperature of the catalyst, to take advantage of the phenomena that, within a predetermined temperature range, conversion efficiency of the catalyst will vary. While this arrangement has certain advantages of the mechanical systems discussed (i.e. less mechanical complexity, simpler retrofit, no moving parts, potential of decreased pressure loss, and the like), certain disadvantages are readily apparent (i.e. energy cost, capital expense, the cost of the surface temperature heating and cooling means, the replacement of components of a temperature responsive in-duct catalyst assembly may be more costly than replacing wear components of a mechanical system, and others).
There is therefore a need for an improved approach to sulfur trioxide conditioning of flue gas streams. The present invention fulfills this need, and further provides related advantages.